After upgrading to Typo3 12, message Access Denied.

Annoying. The permissions of the directories are correct. The database is accessible. Everything seems to be in order in the upgrade wizard.

First, I would check in the extensions, for example, in siteconfig, whether the depends in ext_emconf.php are defined correctly.

You can possibly take the extensions out of the folder briefly and try again.

Otherwise, further:
In the file ext_localconf.php, 'TYPO3_MODE' was originally used, which still worked in TYPO3 11 LTS. This contrasts with the official documentation, which already recommends switching to 'TYPO3'. This change became necessary because 'TYPO3_MODE' is no longer supported in newer versions of TYPO3 and can lead to compatibility issues. By adjusting to 'TYPO3', the problem was resolved, allowing the extension to function as expected again. This demonstrates how important it is to regularly align the code with current recommendations and best practices to avoid future issues and ensure compatibility with new TYPO3 versions.